A unique FIV-like lentivirus has been isolated from a Pallas cat.The investigator proposes to isolate an infectious molecular clone of FIV-Pallas which maintains the biological characteristics of the wild-type virus. Multiple genetic determinants probably are responsible for the increased cytopathogenicity and broad host cell range of the virus.These determinants may be located in the LTR (long terminal repeat), polymerase (pol) and/or envelope glycoprotein (env) regions of the viral genome. Therefore, these regions will be used in chimera production. Chimeric viruses will be constructed from FIV-14 (a molecular clone of FIV-Petaluma) and the FIV- Pallas clone. These chimeras will be tested for their ability to produce syncytia and cell death in several cell lines in which FIV-14 normally does not replicate or replicates with little or no cytopathic effect.